The present invention relates to an optical scanning device; more particularly, to a device in which scanning is effected by the reciprocating linear or continuous rotary movement of a ray-deflecting element.
Optical scanning is well-known and has many civilian, as well as military, uses. Present optical scanners are galvanometer-based devices in which an image produced by an optical system is moved across an image plane in the form of a photodetector. This scanning movement is produced by a beam-deflecting element such as a mirror, which is angularly oscillated about an axis perpendicular to the optical axis by a galvanometer-type drive (e.g., moving-coil, moving-iron, moving-magnet drive).
These scanners, also known as galvo-based scanners, suffer from several inherent drawbacks, the most serious one of which is due to the substantial beam excursion across the system aperture, owing to the fact that the oscillating mirror usually constitutes a pupil of the optical scanning system. This produces serious optical aberrations such as coma, distortion and field curvature. To compensate for these aberrations, a special optical system, such as an Fxe2x88x92xcex8 lens and/or a flattening lens, and electronic means such as non-linear scanning, are required. An Fxe2x88x92xcex8 lens satisfies the condition yxe2x80x2=fxc3x97xcex8; that is, the image height equals the product of the focal length of the objective and the scanning angle. Such objectives exist and are known as F-Theta objectives. A flattening lens is usually located close to the scanned plane and corrects field curvature relative to the required flat field. These optical systems are of great complexity, comprising several aspherical surfaces, provide at best no more than average resolution, and make the scanners very expensive devices.
A second disadvantage of the known galvo-based scanners is the relatively large mass and, consequently, large inertia of the oscillating system, which, especially with wide beams, strictly limits the scanning frequency.
It is thus one of the objects of the present invention to provide an optical scanner which uses a scanning system in which the scanning movement of the beam-deflecting element is linearly reciprocating or continuously rotating, and is thus free of the optical aberrations inherent in the prior art scanners, permitting the use of relatively simple, low-cost optical systems.
It is another object of the invention to provide a optical scanner in which the scanning action takes place in close proximity to the image plane or intermediate image plane, or, in case of a reversed configuration, the plane of the object in the form of a light source.
It is a further object of the invention to provide, in the galvanometer-based configuration, a scanner that has a relatively low inertial mass, permitting scanning at relatively high frequencies and, in the continuously rotating configuration, a scanner that is capable of a relatively high duty cycle.
According to the present invention, the above objects are achieved by providing a device for optical scanning, comprising at least one reflecting element whereby an incident beam of light undergoes at least two reflections; at least one optical system comprising an objective capable of forming an image of an object, said objective, depending on the respective direction of said light beam, defining a pre-objective scanning space and a post-objective scanning space; a mounting structure for said at least one reflecting element; drive means for causing said mounting structure to perform a movement; a light-detecting element for detecting said incident beam of light, or an element for producing light, wherein, with light-detecting, scanning takes place in said post-objective space, and with light-producing, scanning takes place in said pre-objective space.